This invention relates to extraction processes in which two or more elements or compounds are co-extracted with a different degree of intensity. It also relates to an improvement in processes for recovering pure copper and pure nickel from aqueous leach liquors containing these values and at least trace concentrations of metallic impurities.
Aqueous, typically ammoniacal leach liquors containing retrievable quantities of copper and nickel often also contain at least trace amounts of cobalt and molybdenum as well as other metallic impurities such as alkali metal ions. Such liquors may be obtained, for example, by processing manganese nodules with hydrometallurgical techniques or by in-situ mining techniques. A typical source of a leach liquor (pregnant liquor) which contains copper, nickel and other values is set forth in U.S. Pat. No. 3,983,017 to Lester Szabo entitled "Recovery of Metal Values From Manganese Deep Sea Nodules Using Ammoniacal Cuprous Leach Solutions", the teaching of which are incorporated herein by reference.
A process by which copper metal and nickel metal may be recovered from such a solution is disclosed in detail in U.S. Pat. No. 3,855,090 to R. R. Skarbo (Dec. 17, 1974), the disclosure of which also is incorporated herein by reference. The process of the Skarbo patent may be understood with reference to FIG. 1 of the drawing of this application. In the copper and nickel extraction stage 2, a typically ammoniacal pregnant leach liquor enters at 1 and exits at 3 with a depleted nickel and copper content. The nickel and copper and some ammonia is taken up by an organic phase solubilized extractant such as an oxime which enters stage 2 at 4 and exits as a nickel and copper loaded organic stream 5. In addition to copper and nickel, the oxime in stream 5 inevitably contains small but (as will be explained below) significant quantities of impurities. The organic phase oxime is then scrubbed of ammonia at stage 6, and a substantially ammonia-free copper and nickel loaded oxime solution enters the nickel recovery circuit 7 via stream 8. Of course, if the oxime contains no ammonia, such as when a leach solution other than an ammoniacal solution is employed in the extraction stage, the scrub stage 6 can be eliminated.
In nickel recovery circuit 7, the metal loaded oxime stream is contacted in a plurality of stages 10, 11, 12, 13, 14 and 15 in countercurrent flow with a metal-barren acid stream 16. Selective nickel stripping (to the substantial exclusion of copper) is effected by maintaining the mole ratio of the hydrogen ions in the fresh aqueous acid to exchangable nickel ion in the fresh organic stream between about 1.8 and 1.2. The pH of the loaded acid stream 23 is maintained between about 1 and 4. Optimally, the temperature should be between about 40.degree. C. and 55.degree. C. Nickel is recovered from loaded stream 23 in nickel electrowinning stage 30 from which the substantially nickel-barren aqueous stream 16 is collected and recycled to stripping stage 15.
Organic stream 29 loaded with copper exits from stage 15 and is directed to a copper recovery circuit 9. In this circuit, the organic phase is stripped of metal (copper) in stage 32 by acid under conditions of pH and volumetric ratio of organic stream to aqueous stream to exchange hydrogen ions of the acid with copper ions bound by the oxime. From stage 32, an acidic copper-rich liquor exits the stripping stage at 34. Copper is electrowon in stage 35. The now copper and nickel barren organic stream exits the copper stripping stage via line 36 and may be recycled (as stream 4).
From the foregoing description it will be appreciated that nickel recovery circuit 7 and copper recovery circuit 9 produce nickel and copper cathodes. Impurities necessarily build up in these circuits. Thus, trace impurities taken up by the extractant and subsequently stripped collect in both recovery circuits 7 and 9. Thus, impurities can reach levels high enough to contaminate the cathodes or interfere with their production. Furthermore, the nickel stripping in circuit 7 is not complete, and small concentrations of nickel are stripped together with the copper in circuit 9. Since copper is won in stage 35, nickel concentration increases in this stage as electrowinning proceeds. Generally, the nickel ion concentration in copper recovery circuit 9 should not go above about 20 g/l.
A prior art technique for overcoming the problems of impurity buildup simply was to take a bleed stream from each circuit, typically from the downstream side of the electrowinning cells as is shown in FIG. 2. In addition to impurities, the bleed streams contain finite amounts of valuable copper and nickel and other reagents which were lost. Furthermore, various precipitation techniques were required before the bleed streams could be discarded, and these techniques involved ongoing material costs and an investment in original equipment.
In accordance with the Skarbo patent (FIG. 1), a partial solution to this problem was provided by furnishing a cross-over bleed stream 38 from the downstream side of the copper electro-winning stage 35 in circuit 9 to the downstream side of nickel electrowinning state 30 in circuit 7. The provision of such a bleed stream enabled the nickel concentration to be kept at tolerable levels in the copper recovery circuit and optimize nickel recovery. However, various impurities nevertheless built up at least in circuit 7, and a bleed stream 43 and purification system with attendant costs was nevertheless required.